The present invention relates to a turbojet engine exhaust nozzle having a variable geometry.
Modern jet aircraft, which are often required to carry out sub-sonic as well as supersonic flights, may be equipped with an afterburner in the exhaust gas duct upstream of the exhaust nozzle. In turbojet engines without afterburners, the nozzle may be either of a fixed, or a variable geometry type wherein the cross-section of the exhaust nozzle may be varied. However, afterburner equipped turbojet engines must be able to operate either with or without afterburning, depending upon the flight conditions. The cross-sections of the nozzle are quite different for the two operational configurations and, hence, nozzles on such turbojet engines must be of the variable cross-section type.
The first variable cross-section nozzles comprised a variable position convergent segment in which the cross-sectional configuration was easily adjusted as a function of the flight conditions. Presently, in order to achieve higher thrust than may be obtained from a variable converging segment nozzle, the nozzles must have a variable cross-section converging-diverging structure. However, such variable geometry converging-diverging nozzles typically entail complex kinematics and add a significant amount of weight to the aircraft structure.
French Patent No. 1,225,736 discloses a typical structure for a variable geometry converging-diverging nozzle. The controls for the upstream and downstream movable flaps comprise hydraulic actuators axially displacing annuli connected by link rods, or by hinging systems to the flaps, the outer-ring flaps being connected to the inner-ring flaps by link rods. This control system requires two control loops, but each set of actuators must generate sufficient forces to compensate for the pressures exerted by the exhaust gases on the movable flaps. Accordingly, the control means of this type of nozzle are complex and comparatively heavy.